The present invention relates to a device for measuring a physical
force, and more particularly, to a digital load cell weight using a plurality of
load cells for measuring weight.
According to a conventional digital load cell weight, the weight
values detected by load cells are respectively amplified, and converted into digital
data, via analog-to-digital converters. The digitized detected weight values are
then stored in memories. The values stored in the memories are summed to calculate
total weight data, and the total weight data is sent to a total memory.
According to the above conventional configuration, however, when
the digitized detected values involve low frequency noises, problems occur. More
specifically, even if no load is applied to the load cell weight, when each detected
value of the load cells involves low frequency noises, noise-affected waveforms
as shown in Figs. 3A to 3D are input to the memories. If the values read from
these memories are summed, a waveform of data stored in the total memory, which
has to be normally zero level for zero weight, becomes one as is shown in Fig.
3E. Such a waveform will degrade the accuracy of weight measuring.
It is accordingly an object of the present invention to provide a
physical force measuring device, typically a load cell weight, which can avoid
degradation in the measuring accuracy due to low frequency noises.
To achieve the above object, a measuring device of the invention
includes a plurality of load cells; analog-to-digital converting means for converting
detection values, output from the load cells, into digital detection values; digital
filters for respectively suppressing low frequency noise components in the digital
detection values; and arithmetic operation means for summing outputs of the digital
filters to provide weight data.
This invention can be more fully understood from the following detailed
description when taken in conjunction with the accompanying drawings, in which:
- Fig. 1 is a block diagram showing a load cell weight according to an embodiment
of the present invention;
- Fig. 2 is a perspective view of the load cell weight;
- Figs. 3A to 3E illustrate waveforms of low frequency noises which vary with
- Fig. 4 is a block diagram showing a load cell weight according to another embodiment
of the present invention; and
- Fig. 5 is a block diagram showing a load cell weight according to still another
embodiment of the present invention.
Preferred embodiments of this invention will be described with reference
to the accompanying drawings. In the description the same or functionally equivalent
elements are denoted by the same or similar reference numerals, to thereby simplify
In the following, a load cell weight according to an embodiment of
the present invention will be described with reference to the accompanying drawings.
Here, the load cell weight represents a device for measuring a physical "weight."
Fig. 2 shows a configuration wherein four load cells are employed.
In the figure, the reference numeral 11 denotes a rectangular base body made of
an aluminum or the like. Load cells 1a, 1b, 1c, and 1d are respectively located
at the four corners of base body 11. Load cells 1a, 1b, 1c, and 1d are set at distortion
generating bodies (Roberval mechanism) each having a nearly rectangular figure.
The distortion generating body is provided with an 8-figure like through hole at
the side thereof. On the thin top-plate over the through hole of each of load
cells 1a, 1b, 1c, and 1d, two pairs of strain gauges 12a, 12b, 12c, and 12d are
Lower ends of load cells 1a, 1b, 1c, and 1d are mounted at mounting
portions 14a, 14b, 14c, and 14d, respectively, which are fixed at base body 11.
Other lower ends of load cells 1a, 1b, 1c, and 1d face overweight stoppers 13a,
13b, 13c, and 13d, respectively. Other upper ends of load cells 1a, 1b, 1c, and
1d are provided with projections 15a, 15b, 15c, and 15d, respectively. The top
end of each of projections 15a, 15b, 15c, and 15d abuts on the inner plane of load
plate 16 which has an open-box figure and whose size is slightly larger than the
size of base body 11. Thus, load plate 16 can freely move along the vertical direction.
Base body 11 is further provided with enclosure cases 17, 18a, 18b,
18c, and 18d.
In addition, the rear sides of mounting portions 14a, 14b, 14c, and
14d, fixed at base body 11, are provided with variable-height legs 19a, 19b, 19c,
and 19d, by which base body 11 can be kept horizontal.
The above load cells 1a, 1b, 1c, and 1d are respectively connected
to electronic circuits shown in Fig. 1.
Detection values a1a, a1b, a1c, and a1d from load cells 1a, 1b, 1c,
and 1d are respectively amplified through amplifiers 2a, 2b, 2c, and 2d. Amplified
analog values a2a, a2b, a2c, and a2d are respectively converted into digital values
d3a, d3b, d3c, and d3d, via analog-to-digital converters (ADCs) 3a, 3b, 3c, and
Analog-to-digital converters 3a, 3b, 3c, and 3d are coupled to microcomputer
system 4. Microcomputer system 4 includes memories (such as RAMs) 5a, 5b, 5d, and
5d which are respectively connected to analog-to-digital converters 3a, 3b, 3c,
and 3d, and store digital values d3a, d3b, d3c, and d3d. Low frequency noise components
involved in digital values d5a, d5b, d5c, and d5d, respectively read from memories
5a, 5b, 5d, and 5d, are reduced by digital filters (or digital noise reduction
circuits) 21a, 21b, 21c, and 21d, individually.
Filtered outputs d21a, d21b, d21c, and d21d from digital filters
21a, 21b, 21c, and 21d are summed at adder 6, so that the total weight value is
calculated. Total output d6 from adder 6 is sent to total memory 7.
Incidentally, microcomputer system 4 is connected with display 22
for displaying weight values, etc. and key board (10-key) 23 for inputting data.
The embodiment of Fig. 1 will operate as follows.
An object body (not shown) is placed on load plate 16 when weight
measurement is performed. The strains caused at respective load cells 1a, 1b, 1c,
and 1d are detected as analog signals a1a, a1b, a1c, and a1d. The detected values
are respectively amplified via amplifiers 2a, 2b, 2c, and 2d, and then are converted
into digital signals d3a, d3b, d3c, and d3d via analog-to-digital converter 3a,
3b, 3c, and 3d. Digital signals d3a, d3b, d3c, and d3d thus converted are stored
in memories 5a, 5b, 5c, and 5d. Low frequency noise components involved in the
digital signals read from memories 5a, 5b, 5c, and 5d are respectively and independently
reduced by digital filters 21a, 21b, 21c, and 21d. Substantially noise-free digital
values d21a, d21b, d21c, and d21d are summed at adder 6, and summed value d6 is
stored in total memory 7. Value d7 (weight data) stored in memory 7 is displayed
at display 22 in terms of "weight."
When a price corresponding to the measured weight is to be displayed,
a price for a unit weight is input from 10-key 23. In response to data of the 10-key
input and calculated weight data, CPU 8 calculates the product (d7 × d23)
of the 10-key input with the measured weight, and the calculated product as well
as the measured weight are indicated at display 22.
According to the embodiment as mentioned above, various low frequency
noises as shown in Figs. 3A to 3D, which are different for respective load cells,
are independently reduced by digital filters 21a, 21b, 21c, and 21d. Consequently,
the noise reduction effect of the embodiment is superior to a case wherein only
one filter is used for data in the total value as is shown in Fig. 3E.
According to the present invention, low frequency noise components,
involved in detected values which are digitized through analog-to-digital converting
means from the outputs of respective load cells, can be independently reduced by
digital filters. Thus, the high performance noise reduction ensures a stable and
accurate weight measuring.
In the embodiment of Fig. 1, four load cells are used. However, the
number of the load cells may be three, or more than four.
Each characteristic of respective digital filters can be determined
as follows: First, in the actual device according to an embodiment of the invention,
the pattern of noise components as shown in Figs. 3A to 3D is obtained by experience.
Second, the tendency of the actual noise pattern for each load cell is detected
from the result of the experience, so that the filtering parameters of each digital
filter is determined. Then, a combination of respective digital filters provides
a high performance noise reduction which is superior to the performance of only
one digital filter.
The above-mentioned digital filters can be replaced with digital
noise reduction circuits 21a*-21d* shown in Fig. 4.
Digital noise reduction circuit 21a* performs the following operation,
when the noise as is shown in Fig. 3A should be reduced.
More specifically, output d3a(t1) of ADC 3a obtained during a certain
period (t1) is stored in memory (RAM) 5a and digital noise reduction circuit 21a*.
During next period (t2), output d3a(t2) of ADC 3a is stored in memory 5a and is
sent to circuit 21a* in which average d3a(t12) of preceding output d3a(t1) and
new output d3a(t2) is calculated. Further, during subsequent period (t3), output
d3a(t3) of ADC 3a is stored in memory 5a and is sent to circuit 21a* in which average
d3a(t123) of preceding average d3a(t12) and new output d3a(t3) is calculated.
The above average calculations are repeated (i.e., output d3a is
integrated with time). Then, random noise components of all periods are reduced
with a rate of N (N is the number of the average calculations), while actual weight
data, which is not a random variable, is not reduced with the above average calculations.
Each of remaining outputs d3b, d3c, and d3d is subjected to the above
Sum d6 of the above averaged outputs is naturally low noise. However,
if the noise has to be further reduced, another digital filter 21e may be additionally
used for sum d6. Or, as shown in Fig. 5, one digital noise reduction circuit 21e*
may be additionally used for sum d6 of outputs d21a-d21d from the digital filters.
The configuration of a load cell weight should not be limited to
that shown in Fig. 2. Another configuration of a load cell weight, for example,
as is disclosed in Japanese Patent Disclosure (kokai) No. 55-3520, can be used
for the weighing device of the present invention.
The following U.S. Patent discloses a relevant art with respect to
digital processing of the present invention:
U.S. Patent No. 4,660,160 issued on April. 21, 1987 (Tajima et al.),
"ELECTRONIC WEIGHING DEVICE HAVING LABEL PRINTER WITH DATA STABILITY CHECK".
All disclosures of the above U.S. Patent are now incorporated in
the specification of the present invention.
While the invention has been described in connection with what is
presently considered to be the most practical and preferred embodiment, it is to
be understood that the invention is not limited to the disclosed embodiment but,
on the contrary, is intended to cover various modifications and equivalent arrangements
included within the scope of the appended claims, which scope is to be accorded
the broadest interpretation so as to encompass all such modifications and equivalent